Field synthesis and optical subsectioning for standing wave microscopy

ABSTRACT

In an improved optical microscope for observing a luminescent specimen, the specimen is excited by a single, on axis or multiple superposed series of standing wave fields. Then an image of the specimen is recorded and displayed. This specimen can be incrementally moved and additional images can be recorded and processed. Images of the specimen recorded when there are nodes or antinodes at the focal plane of the microscope can be combined by image processing to produce an improved image or set of images of the specimen. Also disclosed are improved standing wave microscopes having a phase conjugator, a transmitted light source, feedback stabilization, an extended light source for field synthesis or a beam contractor. A multiple wavelength light source can be used to view a specimen marked with multiple luminescent dyes.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of our U.S. patent application Ser. No.08/014,328, filed Feb. 5, 1993 now U.S. Pat. No. 5,394,268.

BACKGROUND OF THE INVENTION

The invention relates to luminescence optical microscopy andparticularly to an apparatus and method for selectively opticallyexciting luminescence in particular zones in a specimen such as abiological cell or tissue.

FIELD OF INVENTION

Optical microscopy using fluorescence tagging for the determination ofthree-dimensional structure of cells and tissues is an importantdiagnostic and research procedure. There are a great number of dyeswhich can be attached to various structures within the cell. Whenexcited by a particular wavelength of light these dyes will fluoresce orphosphoresce. For example, several common dyes will emit a red glow whenexcited by green light. Hence, one can see structures to which afluorescent dye is attached. The presence and location of the taggedstructures can provide important diagnostic and structural informationfor basic research and clinical diagnostics.

Fluorescence imaging, in particular, is of vast utility in cell biologybecause of the high specificity of fluorescently labeled proteinanalogs, antibodies, hybridization probes, enzyme substrates, lipidanalogs, and peptides, as well as stains. Fluorescence micrographs ofextremely complicated objects such as intact cells typically showclearly the distribution of the tracer molecules, all other componentsbeing "invisible". The important optical characteristics of manybiological specimens that allow for this simple interpretation of theimage field is that cells are generally weakly refractive and weaklyabsorptive objects.

Conventional microscope images contain information about the 3-Dstructure of the object when the depth of field of the lens system issmaller than the axial dimension of the specimen. This means that in asingle image, the axial location of a particular feature is encoded byits degree of defocus. A 3-D image data set, which is a "zero-order"estimate of the true structure of the object, is obtained by recording aseries of images as the object is stepped through the focal plane of themicroscope, a procedure known as optical sectioning microscopy (OSM).Each image is a spatially filtered axial projection of the object, andeach generally contains in-focus and out-of-focus features. One of thecentral problems in 3-D microscopy is the removal of out-of-focusfeatures from the 3-D image by optical and image processing methodsthereby deriving a refined estimate of the true object.

In fluorescence microscopy there is a linear relationship between theemitter distribution in the object and the intensity distribution in theimage field. This is caused by the mutual incoherence of fluorescenceemission. Dye molecules in the specimen radiate independently so thatthe individual intensity fields are simply superimposed in the imageplane. It is possible to deconvolve the 3-D image to attenuate theout-of-focus portions. However, for the various methods that have beenproposed there is a trade-off of recovery of high-resolution structurefor accuracy or stability.

The alternative to computational refinement of optical sectioning imagedata is confocal scanning fluorescence microscopy (CSFM) in which directoptical spatial filtering is used to remove out-of-focus light wavesfrom the detector field. In one version of this type of instrumentfluorescence is excited in the specimen by a highly focused beam. In theimage plane of the microscope a pin hole is placed at the pointoptically conjugate to the focal point of the beam and a high gain, lownoise detector is placed behind the pin hole. The microscope acts as aspatial filter that detects efficiently only fluorescence photons thatoriginate near the beam focus. 3-D image data is obtained by rasterscanning of the beam relative to the specimen, either optically ormechanically, and stepping the specimen axially through the focal planeto get stacked images. Confocal methods have several shortcomings. Forexample, such images often have a low signal to noise ratio. Hence, theresolution of the image is often severely compromised. Also, scanningusually is relatively slow, with scan times up to 64 seconds per framefor high signal-to-noise images. Indeed, there are many circumstances inwhich this technique cannot be utilized.

For fundamental reasons, a fluorescence microscope is more severelylimited in axial (depth, or inter-image plane) resolution, as opposed totransverse (in the image plane) resolution. Consider a microscope with alens having a high numerical aperture (NA) and a specimen of refractiveindex n illuminated by a light beam having a wavelength λ. Thewell-known Rayleigh resolution formula, 0.61λ/(NA), sets transverseresolution at about 0.2 μm via direct imaging. This can be halved, inprinciple, by confocal scanning. In comparison, the axial equivalent ofthe Rayleigh formula, 2nλ/(NA)², is in the range 0.7-0.9 μm, typical forhigh-quality fluorescence OSM image sets. Computational image processingor confocal scanning can reduce this to 0.4-0.5 μm. A more restrictiveanalysis, the Rayleigh quarter-wave criterion, λ/8n sin² (1/2 sin⁻¹NA/n), gives a theoretical axial resolution in the range 0.13-0.17 μmfor the best microscope lenses. This has been demonstrated intransmitted light microscopy, but not in fluorescence, due to the lackof mutual coherence in fluorescence imaging, and the generally lowersignal-to-noise level. Therefore when the specimen contains finestratified structural features, or simply when it is thinner than thedepth-of-field, fluorescence OSM or even CSFM is unable to yieldsignificant 3D information.

U.S. Pat. No. 4,621,911 discloses a method and apparatus called standingwave luminescence or fluorescence microscopy (SWFM) in which a specimenis illuminated in a fluorescence microscope by means of a standing wavefield at the excitation wavelength. This field is preferably produced bycrossing two equal amplitude coherent beams from a laser. The directionof the beams is such that the nodal and anti-nodal planes in this fieldare parallel to the object plane of the microscope. Under this conditionfluorescence is excited in laminar zones in the specimen, maximally atthe location of each anti-nodal plane. One of these planes can be madecoincident with the in-focus plane. In this way in-focus features of thespecimen are made brightly fluorescent. Immediately adjacent featuresabove and below the in focus plane are in nodal zones and are,therefore, only weakly fluorescent.

U.S. Pat. No. 4,621,911 teaches a theory and embodiments for creationand manipulation of a periodic standing wave field superimposed with thespecimen in a fluorescence microscope, and that sets of images obtainedby standing wave excitation contain Fourier coefficient information onthe axial (depth) structure of the object down to an axial resolutionlimit of λ/4n, as small as 0.068 μm. The embodiments of the patentinclude several methods for producing a standing wave field by crossingtwo equal-amplitude collimated s-polarized coherent beams atcomplementary angles relative to the axis of the microscope. Theseembodiments include the use of total internal reflection (TIR), a mirroror prism, or a wavelength-selective high reflector to fold a laser beamin the specimen region of the microscope, independent coherent beamsentering the specimen from opposite sides, or a re-entrant beam thatfirst emerges from the objective lens into the specimen. The patent alsoincludes embodiments where the nodal planes are not parallel to theobject focal plane although the parallel condition is of principalinterest here. Nevertheless, the method and apparatus of the '911 patentdo not overcome the problems associated with the presence of out offocus luminescent portions in the image.

SUMMARY OF THE INVENTION

We provide improved methods and apparatus which overcome the out offocus problems and clearly display the transverse and axial position ofluminescent structures in the specimen. As in the apparatus of U.S. Pat.No. 4,621,911, a fluorescence microscope is equipped with an opticalsystem for standing wave excitation of the specimen. In a first presentpreferred method we manipulate a single standing wave field to show theaxial structure of a specimen, even when the specimen is so thin that itis entirely within the depth-of-field of a high resolution microscope.We call this extension optical subsectioning, and have found a practicalaxial resolution limit of λ/8n (approximately 0.05 μm) in thinspecimens. In a second present preferred embodiment a nonperiodicexcitation field is generated in the specimen such that the excitationintensity is peaked only at the object focal plane. In this case,intersecting beams entering the specimen from opposite sides are sweptin angle (or multiple sets of such beam pairs are superposed) togenerate a continuous series of standing wave fields that differ in nodespacing, but all with an antinode at the object focal plane. A singleimage is recorded with the swept or multiple-beam excitation, with thenet result being the preferential excitation of structures in thespecimen that lie in the in-focus plane. This differs from the method ofthe '911 patent in that the net excitation field is peaked only at thein-focus plane, and not at evenly-spaced antinodal planes. We call thismethod excitation field synthesis (EFS) or field synthesis fluorescencemicroscopy (FSFM). It represents an extension of standing wavemicroscopy theory and practice to the high-aperture limit, where thedepth-of-field of the optics is less than the axial dimension of thespecimen.

In our method and apparatus we direct two beams, which can be a singlebeam reflected back through a specimen, to create distinct nodes andanti-nodes such as is taught in U.S. Pat. No. 4,621,911. We prefer todirect the beams through the lens of the microscope. In one embodimentthe beam reflects directly back from a mirror positioned underneath thespecimen. Alternatively, we use an active phase conjugator to generatethe return or re-entrant beam in a standing wave illuminator. The phaseconjugator generates the time-reversed version of the excitation beamthat first passes through the specimen, so that a standing wave field ofhigh spatial modulation is created, even in specimens where the incomingplanar wavefronts have been aberrated due to refractive indexinhomogeneties. In another embodiment we use a beam splitter to createtwo beams from a single light source. The two beams preferably aredirected to enter the specimen from opposite sides.

The microscope can be configured to have two objective lenses. In thisconfiguration, transmitted-light optics can be incorporated into themicroscope along with the fluorescence optics. One objective thenfunctions as a transmitted light condenser.

In another embodiment multiple wave-length illumination is used for aspecimen marked with two or more dyes that respond to differentwavelengths of impinging light.

We also prefer to provide a compact interferometer to detect fringedisplacement and generate a signal to adjust the phase of one or bothbeams to compensate for drift.

Using any of the embodiments of our microscope the specimen isilluminated in a manner previously discussed and an image of thespecimen is recorded in a camera and stored. Then the specimen is moveda selected axial distance. Again the specimen is illuminated and asecond image is created and stored. This process can be continued untila desired number of images of the specimen are created. These images arethen combined by image processing to produce one or more combined imagesof the specimen.

A fully interferometric embodiment of the microscope is also describedin which an extended light source, such as a lamp, having limitedcoherence, is used. This allows for simultaneous generation andsuperposition of standing wave fields, and syntheses of a nonperiodic,peaked field.

Other objects and advantages of the invention will become apparent froma description of the preferred embodiments shown in the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the standing wave illuminationof the prior art wherein a specimen is in a standing wave field ofs-polarized light and on the optical axis of a microscope;

FIG. 2 is a schematic representation illustrating the formation of astanding wave field by total internal reflection of an incident beamfrom a cover glass;

FIG. 3 is a side view of a specimen placed in the object focal plane ofa microscope;

FIG. 4 is a diagram showing a first present preferred embodiment of ourimproved standing wave microscope;

FIG. 5 is an enlarged view of the specimen being illuminated in themicroscope of FIG. 4;

FIG. 6 is a block diagram of a image processor used with the microscopeof FIG. 4;

FIG. 7 is a schematic diagram of the second preferred embodiment of ourmicroscope;

FIG. 8a, 8b, 8c, and 8d are graphs is a graph of overlapping standingwave fields having different nodal spacings;

FIG. 9 is a graph illustrating movement of the nodal plane for opticalsubsectioning;

FIG. 10 is a schematic diagram of another preferred embodiment of ourmicroscope which incorporates a transmitted light source in addition toan expanded laser beam;

FIG. 11 is a diagram showing the preferred orientation for thetransmitted light optics in the embodiment of FIG. 10;

FIG. 12 is a diagram illustrating the operation of the achromat lens tocreate a contracted beam in the embodiments of FIGS. 7, 10 and 13;

FIG. 13 is a schematic diagram of another preferred embodiment of ourmicroscope which utilizes an extended light source for excitation fieldsynthesis;

FIGS. 14a, 14b and 14c are diagrams showing orientation and polarizationof the light source image on the main beam splitter used in theembodiment of FIG. 13;

FIG. 15 is an excitation field synthesis ray diagram for the embodimentof FIG. 13;

FIG. 16 is a diagram of another preferred embodiment which uses a phaseconjugator in a self-pumped arrangement;

FIG. 17 is a diagram of yet another preferred embodiment which uses aphase conjugator in an externally-pumped arrangement; and

FIG. 18 is a diagram of an interferometer connected to a standing wavemicroscope having two objectives wherein the interferometer is used todetect and compensate for beam phase drift.

PRINCIPLES OF OPERATION

In a standing-wave microscope, two plane-wave fields from a laser arecrossed at complementary angles in the specimen volume, where theyinterfere (FIG. 1). When the two fields are s-polarized and of equalamplitude, the resulting interference pattern has an electric fieldintensity that varies only axially, as

    I.sub.ex (z)=I.sub.o  l+cos (Kz+Φ )!

where K=(4πn/λ) cosθ, λ is the wavelength and n is the specimenrefractive index. Fluorescence is excited in the specimen in proportionto I_(ex) (z). The nodes or antinodes of the field, which are planesparallel to the focal plane, have a spacing .increment.s=λ/2n cos θ. Bycontrolling the angle θ, the node spacing can be varied down to aminimum value of λ/2n, when the two beams are counterpropagating alongthe axis of the microscope. By shifting the phase of one of the beams,the relative position of the field planes within the specimen can bechanged, at constant node spacing.

It is straightforward to estimate the axial resolution in SWFM. Twoparticles which are separated axially by half the node spacing can beilluminated alternately by shifting the phase of the standing wavepattern. With blue light excitation and a specimen refractive index of1.33 (water) or higher, λ/4n is 0.09 μm or less. In practice, it hasbeen possible to resolve particles separated axially by a quarter fringe(0.045 μm) in specimens where there is little overlapping structure.Because these distances are considerably less than the depth-of-fieldnormally obtained in fluorescence microscopy, "optical subsectioning" ispossible. That is, in thin specimens that fall entirely within thedepth-of-field of a high numerical aperture (NA) objective lens, axialstructure can be observed purely by calibrated movement of an antinodalor nodal plane through the specimen, with no mechanical refocusingrequired. Even when the specimen is thick, discrete outlying structuremay be discriminated by degree of defocus from in-focus features, sothat optical subsectioning will still be useful. In mathematical terms,standing wave excitation is equivalent to axial modulation of the pointspread function (PSF) which in turn is equivalent to shifting theoptical transfer function (OTF) axially in reciprocal space by adistance equal to the spatial frequency of the standing wave field. Thispermits recovery of spatial frequency information that is absent or veryheavily attenuated under incoherent excitation in conventional opticalsystems.

Optical subsectioning is a subtractive process in which a singlestanding wave field is manipulated to resolve the relative axialposition of two or more stratified structures in a thin fluorescentlylabeled specimen. A thin specimen is one which lies entirely within thedepth-of-field of the microscope and, strictly, within one node periodof the standing wave field, 0.17 μm under typical conditions. Inpractice, the phase of the field is adjusted so that a nodal plane iscoincident with one stratum. The structural features of that stratumthen fluoresce only weakly, and an image is recorded that showsprimarily the structures in other strata, i.e., a subtractive image. Thenodal plane is then moved by a known distance to null the fluorescencefrom a neighboring stratum, and a complementary image is then recorded.In the simplest case, two structures that overlap in a conventionalfluorescence image will each appear distinctly in two standing waveimages, where a nodal plane is located first at one structure, and thenthe other.

A standing-wave microscope is, in fact, a type of interferometer inwhich the dye molecules in the specimen act as the primary detectors ofthe excitation field. In our original design (FIG. 2), total internalreflection was utilized to fold a collimated laser beam at the specimencover glass to form the periodic field, which made the use of a highnumerical aperture (NA) immersion lens difficult. This problem is solvedby use of low-divergence gaussian beams propagating within the apertureof a high-NA lens, giving maximum resolution and light collectionefficiency. In the simplest configuration (FIGS. 4 and 5), the beam 21emerges from the lens 8, passes through the specimen 2, and isback-reflected by a closely-apposed mirror 16. One could use a phaseconjugator in place of a mirror 16. The mirror is moved axially by apiezoelectric drive 14, which causes an equal axial shift of theexcitation field planes through the specimen. The optics are adjusted sothat the gaussian beam exiting the objective 8 contracts slightly to alarge-diameter waist (150 μm) at the mirror, 0.1-0.2 μm beyond thespecimen. The standing wave field is then due to the superposition ofthe gaussian field and its reflection. In this condition, the nodalsurfaces of the unperturbed standing wave field, although curved inprinciple, are flat to better than 1 part 20,000 over the field of view.Therefore, these surfaces are called nodal planes. Nodal planes arealways parallel to the mirror in this system.

A more versatile optical system having a laser 64 for a light source wasalso designed, with objective lenses 8 and 55 positioned on oppositesides of the specimen (FIG. 7). A prism splitter 56 is used toamplitude-divide the expanded beam, so that a low divergence gaussianenters the specimen 2 independently from each side. Since the coherencelength of a 1 m ion laser is as short as 30 mm, the two beam paths inthe microscope are typically matched to within 5 mm. A piezoelectricdrive 74 on a mirror 71, 72 or 73 in one beam path serves to adjust thephase, in this case mirror movement being a full wavelength per cycle ofthe standing wave field. One advantage of the two beam system is thatunaberrated wavefronts enter the specimen on both sides, compared to themirror system where phase errors accumulate on both passes when thespecimen refractive index is heterogeneous. The second advantage of thisconfiguration is that by sweeping the beam angle (θ) in the specimen,standing wave fields of different spatial period can be time-multiplexedin the specimen during the acquisition of a single image. The advantageof this is described below.

The two-beam system provides the means for excitation field synthesis(EFS). The microscope operates like an OSM system, in that fluorescenceimages of the specimen are recorded in an electronic camera as thespecimen is stepped through the focal plane. It differs from OSM in thatfor each image, the specimen will be excited by a continuous sequence ofstanding wave fields that differ in axial node spacing, but all with anantinode located at the focal plane of the microscope. The sequence offields is generated by sweeping the beam crossing angle duringexcitation of the specimen. For every field in the sequence,fluorescence will be excited maximally at the focal plane. Away fromthis plane, the phases of each field differ, so that the excitationintensity averaged over all fields in the sequence will be less.Optionally, a second fluorescence image can then be recorded with thesame sequence of standing wave fields, except that each is adjusted tohave a node at the focal plane. In this case, fluorescence will beexcited minimally at the focal plane, but at similar averaged levelsaway from it. Digital pixel-by-pixel subtraction of the nodal image fromthe antinodal image gives a result for which the effective excitationfield is peaked at the geometric focal plane, but decays to zero aboveand below this plane.

In practice, the sequence of standing wave fields can be generated bysweeping the beam-crossing angle θ through the full range accessiblewithin the aperture of the objective lens and condenser. For awater-immersion system having a numerical aperture of 1.2, for example,the range is ±64°. The synthesized field for the antinodal image willthen be ##EQU1## where K₀ =4πn/λ. For the nodal image, only the constantterm does not change sign, so that the difference image is weighted bythe sum of the Bessel terms. This sum is peaked at the focal plane, anddecays to zero in an oscillatory manner above and below it (FIG. 8). Foran idealized optical system in which the numerical aperture is equal tothe specimen refractive index (NA =n, θ_(m) =90°), the weighing of thedifference image is simply J₀ (K₀ z).

The effect of field synthesis on the OTF can be illustrated directly.Fourier transformation of the above equation gives ##EQU2## which is apiecewise discontinuous function of axial spatial frequency. I_(EFS)(k_(Z)) is convolved with the OSM OTF to give the EFS OTF. Therefore, itcan be seen that for an EFS system operating within the aperture ofexisting high-NA objective lenses, the OSM OTF gets expanded piecewiseinto an extended axial spatial frequency band. The EFS OTF has the sametransverse limit as in OSM, but is significantly extended axially.Purely axial wave vectors with frequencies in the range of K₀ cos θ_(m)to K_(o) are recovered directly, and the bandpass limit extends beyondthis for near-axial wave vectors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention utilizes intersecting beams to create a standingwave pattern. Several methods for creating such a pattern are describedin U.S. Pat. No. 4,621,911. FIG. 1, which was taken from that patent,shows the intersection at a fluorescent or phosphorescent specimen oftwo coherent, collimated, monochromatic beams of light, A and B, of awavelength suitable for excitation of fluorescence or phosphorescence inthe specimen. Rays A represent the propagation of a collimated beam(plane waves) that makes an angle θ with the optical axis of themicroscope. Rays B represent the propagation of a second collimated beamthat makes an angle 180°-θ with the optical axis, and is coplanar with Aand the optical axis.

It is also possible to create a standing wave pattern using reflectionso that the reflected beam intersects the incident beam. This techniqueis illustrated in FIG. 2. A sufficiently coherent light source (notshown) directs a collimated beam C which passes through the specimen 2.That beam strikes the cover glass 4 and is reflected as collimated beamD. Since beams C and D are s-polarized and intersect, a standing wavepattern 5, indicated by dotted lines, is formed. The microscope lens 8is positioned so that its focal point 9 in focal plane 10 is within thestanding wave pattern in the specimen.

When both beams A and B or C and D make the same angle θ with respect tothe optical axis of the microscope, as shown in FIGS. 1 and 2, theantinodal and nodal planes of the standing waves are parallel to thefocal plane. Therefore, fluorescence will be excited in the specimen inlaminar zones that show the axial structure of the object. This can bemost clearly seen in FIG. 3.

A side view of the specimen 2 mounted on a glass slide 6 is shown inFIG. 3. The specimen 2 is under a cover glass 4. The microscope lens 8is positioned so that the object focal plane 10 is within the specimen2. If the specimen is illuminated in the manner shown in FIGS. 1 or 2 aseries of laminar zones 12 will be created within the specimen. The nodespacing (.increment.S) of the excitation field varies with changes inthe wavelength (λ) of the beam and its angle (θ) relative to the opticalaxis 3. That is, (.increment.S) =λ/2n cos θ which is a minimum of λ/2nat θ=0°. The relative position of the nodes and the specimen can bevaried at constant node spacing by shifting the phase of one of thebeams. In practice, total internal reflection was utilized to fold onebeam at the cover glass 4 so as to set up a standing wave as shown inFIG. 2. In the method of FIG. 5, θ is limited by the lens system to amaximum value of θ_(m) =sin⁻¹ NA/n'. In the system of FIG. 2, θ islimited by a minimum value equal to the critical angle at the coverglass 4.

Many cells can be tagged with a dye that is excited by green light tofluoresce red; so we can use a green laser for our light source. Forother dyes we may also use blue, red, yellow and even ultravioletlasers. As shown in FIG. 4 it is also possible to use an incoherentlight source 20 such as a high pressure mercury lamp with a beamcollimator 19, polarizer 22 and bandpass excitation filter 23. For themicroscope 18 shown in FIG. 4 the imaging system is comprised of lenses8 and 28, dichroic reflector 26, emission filter 27, beam splitter 25,eyepiece 24, camera 30, image processor 32 and display 34. Light source20 emits a light beam 21 which is expanded collimated and s-polarized.Then the beam is reflected by dichroic reflector 26. Green light passesthrough lenses 28 and 8, cover glass 4 and specimen 2 until it isreflected by mirror or phase conjugator 16. This causes a standing wavepattern shown in FIG. 5 to be created in the specimen. The incident andreflected green light beams also cause luminescent tags within thespecimen to emit red light 29. Emitted red light passes from thespecimen through lenses 8 and 28 and through dichroic filter 26 andemission filter 27 and is directed by beam splitter or reflector 25 toeyepiece 24 or camera 30 or both. The optics are adjusted so that agaussian beam 21 exiting the objective 8 contracts slightly to alarge-diameter waist (150 μm) at the reflective surface 17 of the mirror16 which is preferably 0.1-0.2 mm beyond the specimen 2. The standingwave field is then created by the superposition of the gaussian fieldand its reflection. In this condition, the nodal surfaces of theunperturbed standing wave field, although curved in principle, are flatto better than 1 part in 20,000 over the field of view. An electroniccamera 30 with image processor 32 and display 34 records and displays animage. The display could be a cathode ray tube or film.

In many circumstances specimens can be tagged with two or morefluorescent dyes which are excited by different wavelengths of light.The microscope of FIG. 4 can be used for specimens marked in thismanner. The wavelength could be changed with a filter or by adjusting orswitching lasers.

The image which is created from the emission of light 29 by theluminescent tag 36 can be enhanced using known image processingtechnology. That image can be further improved using the methodsdescribed herein. Therefore, the image processor 32 should include acentral processing unit 37, read only memory 38, and a random accessmemory 39 as indicated by FIG. 6. Normally, the image will be digitizedusing an A/D converter 40 in the image processor 32 or in the camera 30.The digital image is enhanced by the central processing unit 37according to a program in memory 38. Both the original and enhancedimages can be stored in memory 39. The enhanced image is converted toanalog form by D/A converter 41 for display.

A more versatile optical system with objective lenses positioned onopposite sides of the specimen is diagrammed in FIG. 7. A laser emits alight beam 65 which passes through beam expander 66 and lens 67 topivotable mirror 68 and a scan system 69 for field synthesis. The scansystem 69 contains a movable scan mirror 68 and telescope 58. The beam65 is split by beam splitter 56. A portion of the light is directed bymirrors 71, 72 and 73 through phase control 52, tube lens 53 andobjective 55 through the specimen 2. A second portion of the beam isdirected by prism 54, lens 22, dichroic reflector 26 and objective lens8 through the specimen 2. Light 29 emitted from the specimen 2 passes tocamera 30 or oculars 42 through lens 8, dichroic reflector 26, barrierfilter 46 and tube lens 44. The beam splitter and prism are used toamplitude-divide the expanded gaussian beam, so that nearly flat wavefronts enter the specimen independently from each side. Since thecoherence length of a 1 m laser is as short as 30 mm, the two beam pathsin the microscope are matched to within 5 mm. A piezoelectric drive 74can be provided on a mirror 71, 72, 73 or dichroic reflector 26 in onebeam path to adjust the phase. In this case mirror movement is a fullwavelength per cycle of the standing wave field. One advantage of thetwo beam system is that unaberrated wavefronts enter the specimen onboth sides, compared to the mirror system of FIG. 5 where phase errorsaccumulate on both passes when the specimen refractive index isheterogeneous. The second advantage of the configuration of FIG. 7 isthat by sweeping the beam angle (θ) in the specimen, such as by rotationof mirror 68, standing wave fields of different spatial period can betime-multiplexed in the specimen during acquisition of a single image.If this is done while keeping an antinode fixed at the object focalplane, sweeping provides a means for synthesizing a nonperiodicexcitation field peaked at the in-focus plane of the specimen.

The devices shown in FIGS. 4 thru 7 are particularly useful for twomethods of specimen imaging. In one method, the specimen 2 is excited bya time-multiplexed sequence of standing wave fields that differ in axialnode spacing. This generates a set of wave patterns which ifsuperimposed would look like the patterns shown in FIGS. 8b or 8c. Threewaves 75, 76, and 77 are shown. By superimposing a set of standing wavefields having different node spacing, but all having an antinode at thefocal plane in the specimen an effective field that is peaked at thefocal plane can be synthesized, as shown in FIG. 8b.

Therefore, for every field in the sequence, fluorescence will be excitedmaximally at the focal plane. Away from this plane, the phases of eachfield differ, so that the excitation intensity averaged over all fieldsin the sequence will be less. Optionally, a second fluorescence imagecan then be recorded with the same sequence of standing wave fields,except that each is adjusted to have a node at the focal plane. In thiscase, fluorescence will be excited minimally at the focal plane, but atsimilar averaged levels away from it, as shown in FIG. 8b. Digitalpixel-by-pixel subtraction of the nodal (FIG. 8c) image from theantinodal image (FIG. 8b) gives a result for which the effectiveexcitation field is peaked at the geometric focal plane, but decays tozero above and below the plane as, shown in FIG. 8c. The sum of a seriesof curves, such as the curves 75, 76 and 77, in FIG. 8a, is shown inFIG. 6. In practice, the sequence of standing wave fields can begenerated by sweeping the beam-crossing angle θ through the full rangeaccessible within the aperture of the objective lens and condenser byuse of scan mirror 68.

FIG. 9 illustrates the second method, optical subsectioning. A specimen2 has tagged objects a and b. In the first case 90, a nodal plane ismade coincident with object "a", so that the image will show thefluorescence of "b". In the second case 92, the node has been shifted tothe axial location of "b", so that a second image will then show "a".The process can be easily extended to three or more close stratifiedobjects, with the result that a series of images are obtained whichcontain linear combinations of contributions from each stratum. Lineardigital processing can then be used to extract images corresponding toeach stratum.

Two problems with standing wave fluorescence microscopy were immediatelyevident in our early work. First, the use of total reflection in theilluminator was convenient, but precluded the use of high-apertureimmersion lenses. Second, as described, the standing wave data setconsisted of a large number of images for each specimen focal planeposition, since both θ and Φ could be varied. This made the methodimpractically slow. The present methods and apparatus of excitationfield synthesis reduce the data set to one or two images per focalplane, by multiplexing different standing-wave fields.

With the beams counter propagating on axis (θ=0°), and an excitationwavelength of 514.5 nm, the node spacing in the specimen was determinedto be equal to λ/2n in both the mirror and crossed beam systems. In thefirst case, mirror movement of 0.17 μm corresponds to a shift of onefringe. In the two beam system, external mirror movement of 0.514 μm hadthe same effect.

FIGS. 10 and 11 illustrate an optical system which utilizes both a laserlight source and transmitted light. This system has objective lenses 120and 130 positioned on opposite sides of the specimen as diagramed inFIG. 7. For standing-wave microscopy, a laser 100 emits a light beam 101which passes through beam expander 102. The beam 101 is split by beamsplitting prism 104. A prism 105 adjacent to the beam splitter 104 andmirrors 106 and 107 direct a portion of the light through achromat lens108, polarizer 109, upper Wollaston prism 110 and objective 120 throughthe specimen 122. A second portion is directed by beam splitter 104,penta-prism 124, achromat lens 125, dichroic reflector 126, lowerWollaston prism 127 and objective lens 130 through the specimen 122. Fortransmitted-light viewing of the specimen, light emitted from aconventional light source 140, such as an arc lamp passes through heatfilter 141 and lens 142. Prisms 104 and 105, and mirrors 106 and 107direct the light through achromat lens 108, polarizer 109, upperWollaston prism 110 and objective 120 through the specimen 122. Thatlight as well as fluorescence emitted from the specimen passes throughobjective lens 130, lower Wollaston prism 127, dichroic reflector 126and analyzer 129 or fluorescence filter 128 to a camera 132 or eyepiece134 depending upon the position of mirror 133. A shutter, 143 is closedwhen the transmitted-light optical system is in use, thus preventinglight from lamp 140 from entering the microscope through lens 125.

As in the embodiment of FIG. 7, the beam splitter and prism are used toamplitude-divide the expanded gaussian beam so that nearly flat wavefronts enter the specimen 122 independently from each side. Since thecoherence length of a 1 m ion laser is as short as 30 mm, the two beampaths in the microscope are matched to within 5 mm. A piezoelectricdrive 136 can be provided on a mirror 106 or 107 to adjust the phase. Inthis case mirror movement is a full wavelength per cycle of the standingwave field. This system has all the advantages of the embodiment of FIG.10. It also offers a non-laser light source which is useful for manyapplications.

In FIG. 11 the diagram shows the preferred orientation for thetransmitted light optics in the standing wave microscope system of FIG.10. As indicated by the arrows, the Wollaston prisms 110 and 127 arealigned and the polarizer 109 is normal to the analyzer 129.

FIG. 12 shows schematically how the microscope acts as a beam contractorthereby necessitating prior expansion of the laser beam in expander 102in FIG. 10. The microscope of FIG. 7 operates in the same manner withlenses 22 and 53 in FIG. 7 being comparable to lens 108 in FIG. 12 andlenses 8 and 55 in FIG. 7 corresponding to lens 120 in FIG. 12. Lens 108brings the expanded beam to a focus at the rear focal point 121 ofobjective 120. The beam, therefore, emerges from 120 as anearly-collimated, contracted beam having a diameter equal to themicroscope field of view. Because the standing wave microscope is adirect imaging microscope, the entire field of view is imagedsimultaneously. Therefore, it is necessary that the illuminating beamdiameter equal the field of view. With a Zeiss 100×1.25 NA oil objectivethe field of view is 100 microns and the rear focal length is about 1.6mm. Using the fact that triangles A and B are similar the followingrelation is easily deduced

    expanded beam diameter/achromat focal length=contacted beam diameter/objective rear focal length.

In order to achieve a 100 micron illumination beam without using a beamexpander, it would be necessary to use a 64 mm focal length achromatlens for a laser beam diameter of 4 mm. This is mechanically impossiblewithin the constraints of existing microscope components. Therefore, thebeam expander is used to provide a 1 cm expanded beam which the achromatlens 108 and the objective 120 together contract to the desireddiameter.

FIG. 13 shows another embodiment of our microscope having a fullyinterferometric standing-wave illuminator set-up in which the image of anarrow-bandwidth extended light source 150 (not necessarily a laser) isamplitude-divided on a beam splitter 160 and re-imaged in the rear focalplane of two corresponding microscope objectives 120, 130. The twooptical paths between the splitter 160 and the specimen 122 must bebalanced to well within the coherence length of the light source. Forcontrol of field plane position relative to the common-in-focus plane ofthe two objectives, the relative phase of the two beams is adjusted bysubmicron axial movement of beam splitter 160 as indicated by the doublearrow adjacent the beam splitter 160 in FIG. 13. Because the source isextended, the illumination field at the specimen will consist of thesuperposition of mutually-incoherent standing-wave fields differing innode spacing, but all of which will have the same fringe offset at thein-focus plane. When the relative phase is zero (0 degrees) the netfield in the specimen is peaked at the in-focus plane, dropping tohalf-peak intensity away from the in-focus plane. When the moveable beamsplitter 160 is adjusted so that the relative phase is 180 degrees, thenet field in the specimen is nulled at the in-focus plane, rising tohalf-peak intensity away from the in-focus plane. The 0/180 differenceimage (obtained by digital subtraction) will therefore show mainlyfluorescent features located in a narrow zone about the in-focus plane.The extended light source can be a lamp, a scrambled laser, or a laserbeam rapidly swept across the beam splitter 160 in a pattern.

In FIG. 13 light source 150 emits a light beam which passes through bandpass filter 151, polarizing filter 152 and lens 153 through beamsplitter 154 to beam splitter 160. Bandpass filter 151 providesmonochromation of the broadband source and is not needed with a laserlight source. Polarizing filter 152 is oriented to give s-polarizationof the interfering light at the specimen. Lens 153 forms an image of thelight source on the beam splitter 160. The orientation of the image onbeam splitter 160 and elsewhere in the device is indicated by h - - - t.A portion of the light passes through moveable beam splitter 160, lens161, mirrors 162 and 163, field lens 164, dichroic reflector 165, andobjective lens 120 to specimen 122 on movable stage 169. Moveable beamsplitter 160 is preferably 33% transmissive and 67% reflective. Theportion of the light reflected from beam splitter 160 is partiallyreflected off beam splitter 154 through collimating lens 155. Beamsplitter 154 is preferably 50% reflective, so that the light fluxthrough lens 155 is then equal to that through lens 161. Light passingthrough lens 155 is directed to mirror 156, field lens 125, dichroicreflector 126, Wollaston prism 127 and lower objective 130 to specimen122. Field lenses 125 and 164 each form an image of the light source andsplitter 160 in the rear focal plane of the objectives 120 and 130.Fluoresence emitted from the specimen passes through lower objective130, dichroic reflector 126, fluorescence emission filter 128, and bodytube lens 131 to camera 132. The light source for transmitted lightviewing 144 sends a light beam through bandpass filter 145, lens 146,polarizer 147, dichroic reflector 165, Wollaston prism 110 and objective120 to specimen 122. The transmitted light optical system is completedby objective 130, Wollaston prism 127 and analyzer 129.

For fluorescence viewing we prefer not to use the polarizer 109,Wollaston prisms 110 and 127 and analyzer 129 of FIG. 10. Similarly, weprefer not to use Wollaston prisms 110 and 127 and analyzer 129 of theembodiment of FIG. 13 for fluorescence viewing. The ability to removethese components is indicated by the alternate positions shown in dottedline for these components in FIG. 13. Likewise, fluoresence filter 128would be removed for transmitted light viewing.

The improved optical microscope for viewing/imaging a luminescentspecimen uses interferometic illumination for generating a superpositionof axial standing wave fields in the specimen. In previous embodimentsof the instrument single standing wave fields are used, or a series oftime-multiplexed standing wave fields having different node planespacing. In the time-multiplexed case (FIG. 7), the resulting averagedfield can be peaked at the in-focus plane in the specimen, therebyhighlighting the in-focus features to a greater degree than out-of-focusfeatures. The embodiment of FIG. 13 is a significant improvement overthe use of single standing-wave fields which are axially periodic and isa significant improvement over time-multiplexing. The interferometricimprovement of FIG. 13 permits the use of an extended, low-coherencelight source, such as an arc lamp, to simultaneously provide thesuperposed standing-wave fields. This optical design also allows for theuse of a laser light source. If the laser output is scrambled(de-cohered by moving scatterers or in a vibrating fiber optic), it isequivalent to an incoherent source, but is already essentiallymonochromatic. A laser beam may also be used directly, by rapid scanningof the laser beam in a line across the beam splitter 160 of theinstrument. This second use of the laser is similar to the systemdisclosed in FIG. 7 in which a scanning mirror is used to sweep thecrossing angle of the interfering beams in the specimen. The presentembodiment is much improved because it incorporates a beam splittersystem that ensures balance of the two optical paths for all beamcrossing angles in the specimen. The new arrangement also allows forprecision phase shifting of one beam path relative to the other bysubmicron axial movement of the beam splitter 160. The most importantimprovement, however, is the use of an extended light source, which isbandpass filtered (to adjust the coherence length) and imaged as ans-polarized line ("h - - - t" in FIGS. 13 and 14a) on the beam splitter160 and in the rear focal planes of objectives 120 and 130 (FIGS. 13 and15). Collimator and field lenses in each beam path, 155 and 125 in thelower beam path, 161 and 164 in the upper beam path, re-image the linesource in the rear focal plane of each of the paired microscopeobjectives (FIG. 15). Each corresponding pair of image points (a and a',b and b', c and c' in FIG. 15) is coherent, but there is little or nocoherence between distinct pairs (i.e. a--a' is incoherent with respectto b--b'). Therefore, each standing wave field is formed independently(superimposed without cross-interference) and the result is equivalentto the time-multiplexed case, but with much simpler optics. Because thetwo beam paths are balanced, all of the individual standing-wave fieldshave the same offset at the common in-focus plane of the two objectives.This offset can be controlled by shifting the relative length of the twobeam paths in the interferometer, most easily accomplished by submicronaxial movement of the beam splitter 160. As an alternative to a singlepolarized line source imaged onto beam splitter 160, the full apertureof 160 could be utilized with more complex polarizer and transmittancepatterns such as are created with alternative polarization andtransmittance masks 152a, 152b and 152c shown in FIGS. 14a, 14b and 14c,respectively. Polarization and transmittance masking can be accomplishedby separate masks which perform only one of these functions.

As an alternative to the use of a phase conjugator in place of mirror 16in FIG. 5, the phase conjugator 170 (FIG. 16) can be coupled to thespecimen 122 via other optical elements such as an objective lens 120and field lens 108. The phase conjugator can be operated in theself-pumped mode as shown in FIG. 16 in which expanded laser beam 101drives the conjugator. Piezo 174 moves the conjugator axially to adjustthe position of the standing wave field relative to the specimen 122.The conjugator can also be operated in the externally pumped mode asshown in FIG. 17, in which a laser provides beams 172 directly to theconjugator and the position of the standing wave field relative to thespecimen is adjusted via axial movement of piezo-driven mirror 173. Theadvantage of the embodiments of FIGS. 16 and 17 over the use of theconjugator in place of mirror 16 in FIG. 5 is that phase conjugation ismore efficient when expanded beams are used.

In the embodiments shown in FIGS. 7, 10, 13, 15, 16 and 17 a singlestanding wave field can be created by directing a light beam through thespecimen along a path parallel to the optical axis of the microscope. Amirror is mounted immediately opposite the lens so that collimatedcoherent light emerges on-axis from the lens, passes through thespecimen and is back-reflected from the mirror to produce a singlestanding wave field through the specimen or a beam is split and directedinto the specimen from opposite sides on-axis. The ability to useon-axis light beams in a standing wave microscope has not heretoforebeen recognized. Indeed, all embodiments in U.S. Pat. No. 4,621,911 showoff-axis beams.

The illustrated embodiments were described as using monochromatic light.Inherent in that method is the use of a single luminescent dyeresponsive to that light. It is possible to label a specimen with two ormore luminescent dyes which are excited by different wavelengths oflight. If the specimen is marked with more than one dye, the specimen isfirst excited with a light beam having a wavelength which will exciteone luminescent dye and which will establish a first standing wave fieldhaving a nodal plane coincident with a stratum through the specimen. Animage of the excited specimen is recorded. Then, the specimen is excitedwith a second light beam that has a different wavelength, and excites asecond luminescent dye in the specimen. A second image of the specimenis then made. The two images are then combined by any suitable imageprocessing technique to create a combined image of the specimen. Thiscan be repeated at each focal plane and for any desired number ofluminescent dyes. Furthermore, the entire process can be automated.

In standing wave microscopes of the type here disclosed one mayencounter drift of focus and drift of standing wave field position. Thisis noticeable in a standing wave microscope, but not in a conventionalmicroscope, because of the increased axial resolution in the standingwave microscope. One solution to this problem is to construct amicroscope having extremely rigid mechanical components. This is veryexpensive. Another solution is to use feedback stabilization of theposition of the specimen relative to one objective lens of themicroscope as described in "Feedback-Stabilized Focal Plane Control forLight Microscopes" by Frederick Lanni, Review of Scientific Instruments,Volume 64, Jun., 1993, pp. 1474-1477.

Standing wave field drift can also be corrected by feedback stabilizedcontrol of the phase of one of the two interfering beams. FIG. 18illustrates a present preferred feedback stabilization control device.Drift can originate in any part of the optical train at or beyond thepoint at which the second beam is generated. Therefore, the detection ofdrift should occur as close as possible to the specimen. A practicalmethod can be to split off a portion of the light in the counterpropagating beams as close as possible to where the beams enter thepaired objectives. In FIG. 18 specimen 122 is positioned betweenobjectives 120 and 130. Beam splitters 180 and 188 direct a portion ofthe beams through quarter-wave plates 181 and 189, and prisms or mirrors182 and 190 to a beam combiner 183 and then through linear polarizers184 and 186 to dual photodiode detectors 185 and 187. The componentsenclosed in box 191 constitute an interferometer. Polarizationcomponents 181, 189, 184, and 186 make the interferometerdirection-sensitive by converting the linear polarization of theoriginal beams to right-handed and left-handed circularly-polarizedlight (RHCP and LHCP in FIG. 18). Because the diode detectors 185, 187are made polarization-sensitive by linear polarizers 184, 186, thebrightness of the light received by the photodiodes depends on thedirection and degree of drift. Signals from the photodiode detectors canbe used to adjust the phase of one of the light beams to compensate forany drift. Such compensation is most easily done through activation ofthe peizo 136 in FIG. 10 or movable beam splitter 160 in FIG. 13. Wehave observed that in the mirror configuration of FIG. 5 beam phasedrift is not a problem.

Although we have described and illustrated certain present preferredembodiments of our method and apparatus for field synthesis and opticalsubsectioning for standing wave microscopy, it should be understood thatour invention is not limited thereto, but may be variously embodiedwithin the scope of the following claims.

We claim:
 1. An apparatus comprising:an extended incoherent lightsource; a moveable beam splitter positioned such that light from saidlight source is imaged onto said beam splitter and split into an upperbeam and a lower beam traveling in respective optical paths; twoobjective lenses positionable to cause light beams passing through saidlenses to form standing wave fields identically positioned relative to aplane of focus; and, a plurality of reflective surfaces positioned toorient and balance the respective optical paths of said upper and lowerbeams from the beam splitter to said plane to within the coherencelength of said light beams with respect to each other and reflect saidupper and lower beams through separate ones of said two objective lensesto generate a multiplicity of standing wave fields relative to saidplane.
 2. The apparatus of claim 1 further comprising at least onepolarization mask between the light source and the beam splitter.
 3. Theapparatus of claim 1 further comprising at least one transmittance maskbetween the light source and the beam splitter.
 4. The apparatus ofclaim 1, wherein said incoherent light source comprises a scrambledlaser source.
 5. The apparatus of claim 1, wherein said incoherent lightsource comprises a laser rapidly swept in a pattern across said beamsplitter.
 6. The apparatus of claim 1, wherein said beam splitter ismoveable to shift the phase of one of said upper and lower beam pathwith respect to the other beam path.
 7. The apparatus of claim 1 whereinsaid plurality of reflective surfaces further comprises an odd number ofreflective surfaces for one of said upper and lower beams and an evennumber of reflective surfaces for the other of said beams.
 8. Theapparatus of claim 1 further comprising a wavelength filter positionedbetween said incoherent light source and said beam splitter to filtersaid light being imaged on said beam splitter.
 9. The apparatus of claim8 further comprising a polarizing filter positioned between saidincoherent light source and said beam splitter to filter said lightbeing imaged on said beam splitter.
 10. The apparatus of claim 9 furthercomprising a camera positioned relative to one of said objective lensesto record an image at said plane of focus.
 11. The apparatus of claim 10further comprising an image processor connected to said camera tocombine images recorded by said camera.
 12. A method of producingstanding waves relative to a plane of focus, said methodcomprising:positioning two objective lenses to cause light beams passingthrough different said lenses to form standing wave fields identicallypositioned relative to a plane of focus providing an extended incoherentlight source to emit a light beam; splitting the light beam emitted fromthe light source into upper and lower beams traveling in respectiveoptical paths; and, reflecting the upper and lower light beams to orientand balance the respective optical paths of said upper and lower beamsto the plane to within the coherence length of said light beams withrespect to each other and to direct the upper and lower beam throughseparate ones of said objective lenses to generate a multiplicity ofstanding wave fields relative to the plane of focus.
 13. The method ofclaim 12, further comprising polarizing the light beam to produces-polarized standing waves in the specimen.
 14. The method of claim 12,wherein said method includes recording an image at the plane of focus,said method further comprises:labeling a specimen with a fluorescent orphosphorescent label that can be excited by the standing wave fields andemit light; placing the specimen in the plane of focus; and, recording afirst image produced by the light emitted by the label.
 15. The methodof claim 14, further comprising:shifting the phase of one of the upperand lower beams relative to the other beam; recording a second imageproduced by the light emitted by the label; and, combining the first andsecond images to create a combined image.
 16. The method of claim 15,further comprising polarizing the light beam to produce s-polarizedstanding waves in the specimen.
 17. The method claim 16, wherein:saidsplitting further comprises splitting the light beam to produce anintensity antinode at the plane; and, said shifting further comprisesshifting the phase of one of the upper and lower beams relative to theother beam to produce an intensity node at the plane.
 18. An apparatuscomprising:an extended incoherent light source; a moveable beam splitterpositioned such that light from said light source is imaged onto saidbeam splitter and split into an upper beam and a lower beam; apolarizing filter positioned between said incoherent light source andsaid beam splitter to filter said light being imaged on said beamsplitter; a wavelength filter positioned between said incoherent lightsource and said beam splitter to filter said light being imaged on saidbeam splitter; two objective lenses positionable to cause light beamspassing through said lenses to form standing wave fields identicallypositioned relative to a plane of focus; and, a plurality of reflectivesurfaces positioned to orient and balance the optical paths of saidupper and lower beams from the beam splitter to said plane to within thecoherence length of said light beams with respect to each other and toreflect said upper and lower beams through separate ones of said twoobjective lens to generate a multiplicity of standing wave fieldsrelative to said plane.
 19. The apparatus of claim 18 further comprisinga camera positioned relative to one of said objective lenses to recordan image at said plane of focus.
 20. The apparatus of claim 18 furthercomprising at least one transmittance mask positioned between saidincoherent light source and said beam splitter to filter said lightbeing imaged on said beam splitter.